Enable DHCP client option 26/interface-mtu for Arch to support GRE environments with MTU 1454

[hartwork]

Debian has it enabled:

debian@...:~$ sudo fgrep -RiI mtu /etc/ | fgrep -v certs                                                                                                                                                    
/etc/dhcp/dhclient.conf:        netbios-name-servers, netbios-scope, interface-mtu,

Arch does not:

[arch@... ~]$ sudo fgrep -Ri mtu /etc/ | fgrep -v cert
/etc/dhcpcd.conf:# Respect the network MTU.
/etc/dhcpcd.conf:# Some interface drivers reset when changing the MTU so disabled by default.
/etc/dhcpcd.conf:#option interface_mtu